DP83916 datasheet(20/96 Pages) NSC

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30 Buffer Management (Continued)

when its receive resources have been exhausted The sys-

tem should respond by replenishing the resources that have

been exhausted These overflow conditions (Descriptor Re-

sources Exhausted Buffer Resources Exhausted and RBA

Limit Exceeded) are indicated in the Interrupt Status register

and are detailed as follows

Descriptor Resources Exhausted

This occurs when the

SONIC-16 has reached the last receive descriptor in the list

meaning that the SONIC-16 has detected EOLe1 The sys-

tem must supply additional descriptors for continued recep-

tion The system can do this in one of two ways 1) append-

ing descriptors to the existing list or 2) creating a separate

list

1) Appending descriptors to the existing list This is the eas-

iest and preferred way To do this the system after cre-

ating the new list joins the new list to the existing list by

simply writing the beginning address of the new list into

the RXpktlink field and setting EOL e 0 At the next

reception the SONIC-16 re-reads the last RXpktlink

field and updates its CRDA register to point to the next

descriptor

2) Creating a separate list This requires an additional step

because the lists are not joined together and requires

that the CRDA register be loaded with the address of the

RXpktlink field in the new list

During this overflow condition the SONIC-16 maintains

ownership of the descriptor (RXpktin

use i 00h) and

waits for the system to add additional descriptors to the list

When the system appends more descriptors the SONIC-16

releases ownership of the descriptor after writing 0000h to

the RXpktin

use field

Buffer Resources Exhausted

This occurs when the

SONIC-16 has detected that the Resource Read Pointer

(RRP) and Resource Write Pointer (RWP) registers are

equal (ie all RRA descriptors have been exhausted) The

RBE bit in the Interrupt Status register is set when the SON-

IC-16 finishes using the second to last receive buffer and

reads the last RRA descriptor Actually the SONIC-16 is not

truly out of resources but gives the system an early warning

of an impending out of resources condition To continue

reception after the last RBA is used the system must supply

additional RRA descriptor(s) update the RWP register and

clear the RBE bit in the ISR The SONIC-16 rereads the

RRA after this bit is cleared

RBA Limit Exceeded

This occurs when a packet does not

completely fit within the remaining space of the RBA This

can occur if the EOBC register is not programmed to a value

greater than the largest packet that can be received When

this situation occurs the packet is truncated and the SON-

IC-16 reads the RRA to obtain another RBA Indication of

an RBA limit being exceeded is signified by the Receive

Buffer Area Exceeded (RBAE) interrrupt being set (see sec-

tion 436) An RDA will not be set up for the truncated pack-

et and the buffer space will not be re-used To rectify this

potential overflow condition the EOBC register must be

loaded with a value equal to or greater than the largest

packet that can be accepted See Section 342

35 TRANSMIT BUFFER MANAGEMENT

To begin transmission the system software issues the

Transmit command (TXPe1 in the CR) The Transmit Buff-

er Management uses two areas in memory for transmitting

packets

(Figure 3-11) the Transmit Descriptor Area (TDA)

and the Transmit Buffer Area (TBA) During transmission

the SONIC-16 fetches control information from the TDA

loads its appropriate registers and then transmits the data

from the TBA When the transmission is complete the SON-

IC-16 writes the status information in the TDA From a single

transmit command packets can either be transmitted singly

or in groups if several descriptors have been linked togeth-

TLF11722 – 18

FIGURE 3-11 Overview of Transmit Buffer Management

351 Transmit Descriptor Area (TDA)

The TDA contains descriptors that the system has generat-

ed to exchange status and control information Each de-

scriptor corresponds to a single packet and consists of the

following 16-bit fields

TXpktstatus

This field is written by the SONIC-16 and pro-

vides status of the transmitted packet See Section 3512

for more details

TXpktconfig

This field allows programming the SONIC-16

to one of the various transmit modes The SONIC-16 reads

this field and loads the corresponding configuration bits

(PINTR POWC CRCI and EXDIS) into the Transmit Control

TXpktpkt

size

This field contains the byte count of the

entire packet

TXpktfrag

count

This field contains the number of frag-

ments the packet is segmented into

TXpktfrag

ptr01

This field contains a 23-bit pointer

which locates the packet fragment to be transmitted in the

Transmit Buffer Area (TBA) This pointer is not restricted to

any byte alignment

TXpktfrag

size

This field contains the byte count of the

packet fragment The minimum fragment size is 1 byte

TXpktlink

This field contains a 15-bit pointer (A15 – A1) to

the next TDA descriptor The LSB the End Of List (EOL) bit

indicates the last descriptor in the list when set to a ‘‘1’’

When descriptors have been linked together the SONIC-16

transmits back-to-back packets from a single transmit com-

mand

The data of the packet does not need to be contiguous but

can exist in several locations (fragments) in memory In this

case the TXpktfrag

count field is greater than one and

additional TXpktfrag

ptr01 and TXpktfrag

size fields

corresponding to each fragment are used The descriptor

format is shown in

Figure 3-12

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